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Discussion Starter #1
I've been searching the internet, but haven't come up with an answer. Also, if my 5.5G tank has 20w of CF light, is it medium or high? The lights are just on top of the canopy so that would be around 8" from the substrate's surface. Does the light source height make a difference in small tanks?

Thank you =)
 

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In general, the WPG guideline is outdated anyway, since it was developed with T12 bulbs in mind. With the newer T8, T5 and T5HO bulbs, the WPG guideline is pretty much useless nowadays. However, people still follow it to the book (for whatever the reason), and end up thinking they have a low light tank with 1.5 WPG of T5HO light (which is not the case!)

A better guideline to measure light would be to use PAR, but not everyone has access to a PAR meter so...

Here is an interesting article regarding CF lights, however.

http://www.plantedtank.net/forums/lighting/85667-par-data-spiral-power-saver-bulbs-2.html#post837592
 

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I don't think that in general people follow wpg by the book. When I read posts it becomes a starting point. I see people say "That's about 2wpg. But if that's a T5 then that's high light. T5's put out at least 2xs as much light." I've also read this in reverse of course. I think everyone agrees PAR is the all end all, but what regular hobbyist has a PAR meter? Most clubs don't even own one to share. SO... you have to start somewhere.

WPG breaks down in smaller tanks because... lets say most people think you need 2-3 wgp. 50 gallons -100 -150 wpg. Well if you have one gallon that would mean a light that was only 2-3 watts. You can't grow plants with 2-3 watts of light. There's a point that you can't go below and still grow plants. That's the layman's explanation. For a scientific one go to Rex Griggs site here, minimum threshold for lighting.

http://www.rexgrigg.com/mlt.html
 

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I agree with Tex, IMO wpg is a good starting point, and then you have to factor in the other things that can affect each situation.

The way I understand the small-tank-vs-lighting issue is I think about the sun. It spreads light all through the solar system, but only a tiny fraction actually lands on the earth. Then, you also have to take into account how light will reflect and refract when it passes through glass & acrylic.

A smaller tank has small surface area- so the light coming from most fixtures is going to miss the tank, or be refracted by the tank walls and scattered out beyond the tank. Less light will stay inside a small tank than would a big tank.

I'm sure there's more to it, but this is the bit that sticks in my brain cells. :smile:
 

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It was never more than a coincidence that 2 watts per gallon meant anything at all, and it only meant something for standard 4 foot or 2 foot T12 bulbs, used on standard shaped tanks, like 55 gallon or 29 gallon tanks. Physically, light intensity depends on how far you are from the source of light. Nothing else even comes close to being that important. You could theoretically make a 6"L x 6"D x 36"H, and then you would be very close to having a "light pipe" where the intensity would drop only slightly with distance from the light. That is because much of the light striking the walls of the tank would reflect back into the tank, preventing the normal drop in intensity. But, for any conventional tank, you can ignore all of the dimensions except the height. Each type of bulb, whether T12, T8, T5HO, MH, or PC, even screw in PC bulbs, gives an intensity that varies with the square of the height of the tank, but not with the wattage of the bulbs. So, you can pick the light by the height of the tank, and the length of the bulb or number of bulbs by what it takes to cover the substrate with reasonably uniform intensity. That is true for 1 gallon tanks as well as 100 gallon tanks.
 

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Discussion Starter #7
Wow... very interesting all the info you've given me guys, thank you. But I'm confused. The reason behind the question is that it seems I'm having a nutrient vs light balance problem.

If I have a 16" x 8" x 10" tank with 2 10watt Screw-in compact fluorescent bulbs lengthwise 8" away from the substrate and 1" from the surface of the water, is my tank medium or low light?

I'm sorry to ask, is just that I still can't make a reference for my tank. There was a 19watt bulb in the PAR thread, but I'm not sure if that applies to me since the tank and height of the light were different.

Thanks.
 

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Whether you have high or medium light depends on other factors as well (these were mentioned in the PAR thread I linked to).

What is the orientation of your bulbs? Do you have reflectors?
 

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I don't think this has been mentioned yet, but basically the reason why the wpg rule doesn't work for small tanks (or large tanks) is because light falls on a surface area (two dimensions), while gallons is a measure of cubic space (three dimensions). So wpg is flawed from the start. It's like measuring how fast a car can go by saying it gained 20 feet of height in 10 seconds. Doesn't make sense ... well it does if the car is driving up a hill, but it all depends on how steep the hill was, etc. It would make a lot more sense to measure the speed of the car in km/h or miles per hour. The same goes for light. Since it falls on a surface area, you can only really measure it in light per surface area. Without getting out measuring instruments etc. you can get a decent idea of how much light you have compared to others by using the watts per square inch (or square foot) rule. Way more accurate that wpg. However, like already mentioned the watts a bulb uses results in different amounts of light depending on which technology it's based on (ie: a T12 bulb will use more watts for the same amount of light that a T5 bulb produced). So if you wanna get even more accurate, look up the lumen output of the bulb in question (you can usually find that on the manufacturers website or even on the bulb packaging) and use the lumen per square inch (or square foot) rule. That will be your most accurate measure (without investing in extra equipment, etc.) and it very dependable, because you're basically measuring how much light the bulb puts out and the area that that light falls on. The only thing that can still screw you up is the reflector quality, but to factor that in, you'll need either very complex math or measuring instruments.

Hope this helps clear things up a bit.

Btw, the reason why the wpg rule almost works is because between a certain range of tanks (bigger than very small and smaller than very large ;) ) the ratio between gallons and surface area is fairly constant, but once you get out of that range the ratio changes drastically and the wpg rule breaks down completely.

Harry
 

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Discussion Starter #11
I don't think this has been mentioned yet, but basically the reason why the wpg rule doesn't work for small tanks (or large tanks) is because light falls on a surface area (two dimensions), while gallons is a measure of cubic space (three dimensions). So wpg is flawed from the start. It's like measuring how fast a car can go by saying it gained 20 feet of height in 10 seconds. Doesn't make sense ... well it does if the car is driving up a hill, but it all depends on how steep the hill was, etc. It would make a lot more sense to measure the speed of the car in km/h or miles per hour. The same goes for light. Since it falls on a surface area, you can only really measure it in light per surface area. Without getting out measuring instruments etc. you can get a decent idea of how much light you have compared to others by using the watts per square inch (or square foot) rule. Way more accurate that wpg. However, like already mentioned the watts a bulb uses results in different amounts of light depending on which technology it's based on (ie: a T12 bulb will use more watts for the same amount of light that a T5 bulb produced). So if you wanna get even more accurate, look up the lumen output of the bulb in question (you can usually find that on the manufacturers website or even on the bulb packaging) and use the lumen per square inch (or square foot) rule. That will be your most accurate measure (without investing in extra equipment, etc.) and it very dependable, because you're basically measuring how much light the bulb puts out and the area that that light falls on. The only thing that can still screw you up is the reflector quality, but to factor that in, you'll need either very complex math or measuring instruments.

Hope this helps clear things up a bit.

Btw, the reason why the wpg rule almost works is because between a certain range of tanks (bigger than very small and smaller than very large ;) ) the ratio between gallons and surface area is fairly constant, but once you get out of that range the ratio changes drastically and the wpg rule breaks down completely.

Harry
Thanks Harry. I searched the oceanic website for the lumen info of my bulbs, but wasn't there or the box. :(

Whether you have high or medium light depends on other factors as well (these were mentioned in the PAR thread I linked to).

What is the orientation of your bulbs? Do you have reflectors?
That's true. Well, the bulbs are screwed to a double socket adapter located in the middle of the hood, which is 16" x 4". The hood (and bulbs) rests horizontally directly on top of the middle part of the glass canopy, and there's virtually no space between the bulbs and the canopy. So if any light escapes the hood is doing so through the canopy into the tank below. There's a reflector behind the bulbs, looks like aluminum or something like that, and I covered with aluminum foil what's not covered by the reflector. I doubt that the foil does anything to bounce light back into the reflector, but anyhow... I was trying to get as much light in the tank as possible :p

I'm sorry to keep trying to get an answer from you guys. I understand that you have sent me a lot of info, and probably it should be enough to figure it out, but for a noob like me all this information is very confusing, in the meanwhile my plants are dying and by adding more the problem just gets worst.

I appreciate any kind of help. Thanks :)
 

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Any guidelines on the lumens / area rule? This seems like the most reasonable method of quantifying the amount of light output aside from PAR. I'm considering setting up a planted tank, nano (2.5 gal) DIY on the cheap. Eyeing LEDs, but don't have a good idea of what kind of light output I'll need. High and low light guidelines would be great!
 

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Any guidelines on the lumens / area rule? This seems like the most reasonable method of quantifying the amount of light output aside from PAR. I'm considering setting up a planted tank, nano (2.5 gal) DIY on the cheap. Eyeing LEDs, but don't have a good idea of what kind of light output I'll need. High and low light guidelines would be great!
According to here: http://www.thekrib.com/Plants/Tech/par-moles.html here's some lux (lumens per square meter) numbers:

500 Lx - low light (crypts, java fern)
1000 Lx - moderate light (Anubias, Echinodorus sp.)
1500 Lx - bright (Aponogeton sp., Ludwigia sp.
2500 Lx - very bright (Riccia fluitans, Limnophilia aquatica)

So the first one for example would mean 500 lumens per square meter, however, obviously your tank is way smaller than one square meter. So start with figuring out how much area your tank takes up. I don't remember the dimensions of a 2.5G, but for a 5G, you're looking at 16x8 inches which converted to metric is about 825 square cm, which is 0.0825 square meters. Multiply the lux by the square meters of your tank to get the lumens, so 500 times 0.0825 means you need 41.25 lumen over a 5G tank to get low lights ... although I personally would call this very very low light. At the brighter end, 2500 lux, you'll need 206 lumen over a 5G.

Now you have to remember that the above calculations don't take into account the fact that not all the light from the bulbs actually hits the area we are trying to illuminate. A decent amount of it leaves the tank and never makes it to the bottom where the plants are. So in reality a 206 lumen bulb will probably never ever give you a high light situation, unless maybe you line the tank glass with mirrors and have a perfect reflector, etc.

Just as an example to show you how much light never makes it to the bottom, on my 55G I had two 4 foot T8 bulbs rated for around 2700 lumen each. Total surface area of my tank is 0.3716 square meters. Based on the formulas if all the light I had was to reach the bottom I would have had a lux reading of 14531, or super super super bright. In reality, so much light leaves the tank that I would have classified this tank into the moderate lighting range or (just guessing) 1000 to 1500 lux.

So to sum it all up, best use for the above lux values is with the use of a lux meter since there's no easy formula to figure out how much light is lost to outside the tank.

Harry
 

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Stay away from lux.

I use a PAR meter.
With smaller nano sized tanks, the crappy bulbs they use for 7W, 13 w ranges really are junky garbage. Really.

I think and the meter shows this, that they simply do not put out the same PAr per watt as say a nice 96W or 65 W PC bulb. Reflectors also stink for those junky little cheapo hoods/clpi ons they sell for nano tanks.

I'd say that is where the Watt/gal issue lies, you are using junk to light the tank, and the quality is the problem, not the rule really. If they put out the same PAR per watt, then it would be much more similar.

There are other issues, like good spread w/lower intensity vs hot spotting being more an issue, you do not have the space often times etc, but the PAR is really wimpy from the nano tanks.

With a meter, there's no NEED for measuring or including area.

By measuring various brands and light types, plotting the decay slopes we can easily tell folks what range of light they have without testing their particular tank for most general light questions.
They can also borrow a club meter to see/rent one even........then they know.

I find it rather curious that many beat the snot out of EI whining like babies about the need to test ppm's yet try and suggest NOT to test light, rather, simply use a few areas/watts to ESTIMATE (at best not even a close guess with respect to light).

Pretty hypocritical if you ask me.
If you claim the need/suggest to test, do it right.
Otherwise just live with the estimation/wait till someone test your brand and set up and use their data for a closer range/estimation.

The W/gal rule only seemed to ever break down with small tanks and this was due to junk bulbs/reflectors, not the rule so much itself. In other words, the bulbs caused the issue, not the rule when used with more standard common sizing. I agree some of it is due to how the light is spread over a distance.

Borrow a meter if you are honestly interested. Then you can test and see.

Regards,
Tom Barr








Regards,
Tom Barr
 
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